As one of approaches for realizing a third-generation DNA sequencer, a technology utilizing a nanopore has been widely investigated. In other words, this technology is a technology in which a pore whose size is almost equal to the size of a DNA (nanopore) is punctured through a thin film membrane; chambers located above and below the membrane are filled with aqueous solutions; an electrode is prepared in each chamber so that the electrode gets touch with the aqueous solution in each chamber; DNAs, which are measurement targets, are put in one of the chambers; and the time variation of anion current flowing through two electrodes respectively prepared in both chambers is measured when DNAs are made to pass through the nanopore by electrophoretically accelerating the DNAs owing to the potential difference set between the electrodes, with the result that the structural characteristics and base sequences of the DNAs may be determined. In addition, the above method is useful for obtaining the structural characteristics of not only DNAs but also various biological molecules.
In the manufacture of nanopore devices, because of the high mechanical strength of nanopore devices and other reasons, manufacturing methods using semiconductor substrates, semiconductor materials, and semiconductor processes have drawn much attention. For example, a thin film membrane can be formed using a silicon nitride film (SiN film), and a nanopore whose diameter is equal to or smaller than 10 nm can be formed by reducing the irradiated area of an electron beam on the membrane and by controlling the energy and current of the electron beam with the use of a TEM (transmission electron microscope) apparatus (Nonpatent Literature 1).
In the process using the TEM apparatus, the size of a sample disposed in the apparatus is restricted, and furthermore it is usually difficult to simultaneously reduce the irradiated areas of electron beams for plural positions of a membrane. Therefore it is impossible to simultaneously process all chips on, for example, an 8-inch wafer, and it is necessary to cut off chips from the wafer and to process the chips one by one. Because a nanopore process is executed after the elapse of a long time owing to the vacuuming and stabilization of the TEM apparatus every time a chip is disposed in the apparatus, the throughput of the device formation decreases. In addition, because the drift of the electron beam radiation is virtually unavoidable, the variation of the diameter of a nanopore becomes a few nanometers. Furthermore, it is difficult to form nanopores whose diameters are equal to or smaller than 10 nm using patterning executed by a usual semiconductor lithography technique.
Nonpatent Literature 2 discloses a method for forming nanopores. The authors of Nonpatent Literature 2 disclose a method in which potassium chloride aqueous solutions (KCl aqueous solutions) are disposed above and below with a SiN film 10 nm thick that does not have pores therebetween, two electrodes are immersed in the KCl aqueous solutions of upper and lower chambers respectively, and a high voltage is applied between both electrodes. Furthermore, a certain cutoff current value is set in advance, and when a current between the two electrodes exceeds the cutoff current value, the application of the high voltage is stopped. As shown in FIG. 2(f) in Nonpatent Literature 2, if a voltage of 5 V is continuously applied, the current between the electrodes reaches the cutoff current value in 400 s to 500 s, and by stopping the application of the high voltage at this moment, a nanopore with its diameter about 5 nm is formed.